1. Technical Field
The invention is directed to a method of depositing an oxide coating containing indium on a substrate, more particulary to a process of depositing an oxide coating containing indium and tin on a substrate. The invention is also directed to substrates coated by the method of the invention.
2. Brief Description of Background Art
Transparent, electrically conductive, tin oxide coatings are utilized, for example, in articles such as flat panel picture tubes for television screens, CRT screens or monitors for computers, faceplates for cathode ray tubes, etc. As discussed in "ITO Films: Adaptable To Many Applications" by Dennis R. Nichols, PHOTONICS SPECTRA, May, 1982, it has been reported that films of indium oxide doped with tin oxide, i.e., "ITO" films, produced using vacuum sputtering techniques can provide advantageous optical and electrical properties for such articles. However, while such films produced utilizing vacuum sputtering processes have good electrical conductivity and are transparent to visible light, vacuum sputtering processes are very costly.
It would be desirable to be able to utilize chemical vapor deposition techniques for the deposition of such films. Chemical vapor deposition techniques in general tend to be more adaptable for coating relatively large areas of substrate relatively inexpensively. However, there are a number of disadvantages associated with chemical vapor deposition systems for providing substrates having oxide films containing indium.
For example, U.S. Pat. No. 3,944,684 relates to a process for fabricating transparent, electrically conductive coatings of tin and indium oxides by chemical vapor deposition The "indium chelate" disclosed for use in the process to which the patent is directed is a solid having a relatively high melting point of 173 degrees Celsius and, moreover, is volatilized and transported in an inert gas stream at high temperatures above about 180 degrees Celsius.
Typical apparatus presently in use for chemical vapor deposition systems comprise a bubbler which contains a supply of an organometallic compound chosen for a particular process, a reactor or deposition chamber which contains the substrate on which a film is to be deposited, a source of a carrier gas which is inert to the organometallic compound in the bubbler and either inert or reactive to the compound in the deposition chamber, and optionally sources of other reactants or dopants supplied to the reaction chamber The bubbler and contents are maintained at a constant and relatively low temperature which typically is above the melting point of the organometallic compound but well below its decomposition temperature.
While it is possible to vaporize a sublimable solid organometallic compound in a bubbler, it is difficult to control its rate of vaporization. The surface area of a solid exposed to the carrier gas changes as vaporization proceeds. In contrast, a liquid contained in a bubbler with substantially vertical walls presents the same surface area of a solid to the carrier gas so long as the flow and bubble size of the carrier gas remains steady. Thus organometallic compounds for metal organic chemical vapor deposition desirably are liquids at the bubbler temperature and should have a relatively high vapor pressure, i.e., a vapor pressure of at least 1.0 torr at the bubbler temperature Moreover, the decomposition temperature of such organometallic compounds desirably should substantially exceed the bubbler temperature, but the compounds should react readily (by decomposition or otherwise) at the temperature encountered in the deposition chamber.
U.S. Pat. No. 4,720,560 is directed to organometallic compounds of gallium and indium corresponding to a specified formul MR.sub.x (M being gallium or indium) in which at least two different R substituents are associated with each metal atom. It is disclosed that the ultimate utility for such compounds, employed in metal organic chemical vapor deposition, is to provide a coating of the constituent metal, or (in combination with other reactants introduced in the deposition chamber) to provide coatings of metal oxides, nitrides, III-V compounds, and so forth. At least in part, the patent is directed to providing indium compounds, for example indium compounds having different alkyl substituents on the metal M, which indium compounds desirably are liquids at or slightly above room temperature and have a vapor pressure of at least about 1.0 torr at the bubbler temperature. After discussing the relative melting points, vapor pressures at 30 degrees Celsius and decomposition temperatures of triethylindium and trimethylindium, patentees observe that because of the lack of homology in these indium compounds and the small number of known indium compounds, that those of ordinary skill in the art have been prevented from selecting an optimal compound for indium metal organic chemical vapor deposition.
U.S. Pat. No. 4,847,399 is directed to the preparation or purification of volatile organometallic compounds corresponding to a specified formula, a specific example of which includes trimethylindium, which organometallic compounds will have less total volatile organometallic impurities than before. Under the discussion of background art, U.S. Pat. No. 4,847,399 teaches that volatile impurities are carried into the deposition chamber and thus must be minimized in chemical vapor deposition source compounds. It is further taught therein, that because many interfering solvents and organometallic compounds are volatile, and thus are difficult to separate from volatile Group III-A alkyl compounds by distillation, the problem of purity is aggravated. By way of illustration, the patent refers to the specific example of organic contamination of a Group III-A alkyl compound by the presence of complexed ether in trialkylindium compounds which are prepared in an ether solvent. Thus U.S Pat. No. 4,847,399 suggests that such complexed ethers should be avoided in organometallic chemical vapor deposition.